Chop pistachios into teeny pieces using a mortar and pestle or a food processor. Reserve.

Remove bananas from the freezer and let sit at room temp for about 5 minutes, or until they are slightly less frozen than rock-hard.  Peel bananas (if this was not done before freezing).

Cut bananas into 1-inch pieces and place in a food processor. Puree until bananas are transformed into a smooth mixture. Add pistachios and pulse a few times to incorporate them.

Transfer pistachio banana mixture into a piping bag with a star tip and pipe into cones

Plan-overs
Extra pistachio banana mixture can be frozen, then remixed in a food processor just before serving.